Objective: The objective is to establish the nature and the degree of the modification of the process of ethylnitrosourea (ENU) carcinogenesis by selected drugs. Approach: Groups of 100 mice each were injected intraperitoneally with 60 micron grams/g of ENU in saline at 4 weeks of age. The mice were weaned and fed one of the selected drugs for a period of six weeks and observed thereafter. The drugs chosen are known to alter hepatic catalase activity: aminotriazole (AT) inhibits hepatic catalase, while clofibrate, dimethrin and W-1372 increase hepatic catalase contents.